A power processing module or power converter is a power processing circuit that converts an input voltage or current source into a specified output voltage or current. Power processing modules are typically used in a wide variety of applications to provide a source of regulated power. There is a growing demand, particularly the telecommunication and computer industries, for increased current handling capabilities in power processing modules.
A common approach to increase the current handling capacity of existing DC/DC power processing modules is to add additional power stages to an already existing power module. This distributive approach provides for greater expandability of the power system, permitting the use of lower current distribution buses. Furthermore, a distributed power system with the ability to use standard power processing modules allows for redundancy, which in turn, increases the reliability of the entire power system. Examples of distributed power systems include the use of power modules with their own individual isolated input filters or different input buses. Other distributed systems may employ power modules that share a common input bus or filter, or alternatively share a common output bus, i.e., paralleled outputs, for increased power or redundancy.
Distributed power systems, however, have inherent shortcomings. For example, in a paralleled distributed power system, i.e., the power modules share a common output bus, the power modules' switches are required to be turned on simultaneously, resulting in very large current "steps." These large current steps, in turn, increase the stresses on the input and output filter components, such as the input and output filter capacitors. The resultant impact of the increased stresses is to decrease the overall system reliability. Additionally, the large current steps also increase the electromagnetic interference (EMI) generated by the power converter. To overcome the above-mentioned problems, larger input and output filter components are required to minimize the input and output ripple currents and the EMI thus generated, increasing the component count and cost of the power converter.
The performance of a distributed power system may be improved by synchronizing and/or interleaving the switching frequencies of the individual power modules. When multiple power processing modules are employed in a distributed power system to increase the overall current capacity, it is also desirable to minimize the bandwidth of the switching EMI spectrum by confining the switching frequency of the power converter switches to a single frequency. Reducing the switching frequencies of the power converters to a single frequency also reduces the electrical noise generated by the switches to a single frequency, which places less exacting requirements on the design of the input and output filters. The use of a single switching frequency also eliminates beat frequency interactions caused by differences in switching frequencies.
The typical approach to implementing a single frequency system of power modules is to merely drive all power stages with the same switching drive signal. While synchronization is inherent in the method, the input current drawn from the supply source during the "on" time of the power switch is increased proportionally to the number of power modules in the distributed power system. This large "slug" of current is stressful on filter components and is the source of significant EMI, both of the line conducted as well as the radiated variety. A similar problem manifests itself at the output of the system.
A substantial improvement over the previous technique can be realized by staggering or "interleaving" the drive pulses, with respect to one another, such that the modules are driven out of phase. Synchronization of switching frequency is retained while adding the benefits of interleaving. An auxiliary circuit is used to generate two drive signals which are phase-shifted from one another by 180 degrees or T/2, where T is the period of the switching cycle. This approach, however, requires complex circuitry to ensure strict T/2 operation and is limited to two anti-phase driven signals.
Accordingly, what is needed in the art is an improved method of synchronizing the switching frequencies of power processing modules that mitigates or eliminates the above-described shortcomings.